The present invention relates to an image data structure, an image recording method, an image recording apparatus and an image recording program.
In order to represent a color image, three-channel image components such as R/G/B, Y/Cr/Cb are required. Further, since image data is large in its capacity, image data is generally transferred in a compressed data format by means of a removable memory or a communication line. JPEG is widely spread as the compression processing method. The compressed image data can be expanded at a high speed by a hardware processing using an expansion circuit corresponding to the compression method.
Conventionally, there is known image data in which an alpha channel that is a transmission ratio component for composing two images in a superimposing manner or clipping an image is added to image components of three channels (see JP-A-2001-202504, for example).
However, in many cases, the expansion circuit of image data is exclusively designed for expanding image data. For example, in an expansion circuit for the JPEG schema, there arises a case that image data of four channels can not be expanded even if it is image data of the JPEG format. In this case, the transmission ratio component or both the image components and the transmission ratio component is required to be expanded through a software processing. In this manner, there is a problem that the image data of four channels constituted by the image components of three channels and the compressed transmission ratio component is degraded in general-purpose properties.